 The silver is then recovered by volatalising the zinc.The Parkes process largely replaced the Pattinson process, except where the lead contained insufficient silver, in which case the Pattinson process provided a method to enrich it in silver to about 40 to 60 ounces per ton, at which concentration it could be treated using the Parkes' process.The initial product of copper smelting was impure black copper, which was then repeatedly melted to purify it, alternately oxidizing and reducing it.